Tunable multi-wavelength semiconductor laser array for optical communications based on wavelength division multiplexing

ABSTRACT

Techniques, devices and systems for optical communications based on wavelength division multiplexing (WDM) that use tunable multi-wavelength laser transmitter modules.

CROSS-REFERENCE TO RELATED APPLICATION

The present patent application is a continuation of U.S. applicationSer. No. 12/788,121, filed on May 26, 2010, now pending.

TECHNICAL FIELD

This specification relates to lasers, optical transmitters and opticaltransceivers and their applications in optical communications based onwavelength division multiplexing (WDM).

BACKGROUND INFORMATION

Various optical fiber transmission systems use optical WDM transceiversto transmit and receive data by combining a number of different opticalchannels or signals at different WDM wavelengths onto a single fiber.Light at these WDM wavelengths is modulated as optical signals atdifferent WDM wavelengths to carry data of different signals,respectively. For example, an optical fiber transmission system can bedesigned to include n number of optical WDM channels each with a datarate of m Gb/s to transmit through a single fiber with data throughputrate at n×m Gb/s. As such, data transmission at a data throughput rateof 100 Gb/s can be achieved by using, for example, 10 optical WDMchannels each at a channel data rate of 10 Gb/s or 4 optical WDMchannels each at a channel data rate of 25 Gb/s. To achieve asufficiently high data throughput rate at n×m Gb/s, the number ofoptical WDM channels, n, can be increased to reduce the data rate m peroptical channel to advantageously use relatively matured low-data-rateoptical WDM technologies and the associated CMOS electronic technologiesfor the electronic driver and data processing circuits.

Optical WDM transceivers can be in various configurations where eachtransceiver includes a transmitter part that transmits one or moreoptical WDM signals and a receiver part that receives one or moreoptical WDM signals. An integrated multi-wavelength WDM transceiver is atransceiver in a compact platform that allows multiple streams of datato be simultaneously placed on a single physical input and output (I/O)port using multiple optical WDM wavelengths from an array of lasersoperated at the optical WDM wavelengths. Such integration offers anumber of advantages including low power operation, spatial and costefficiency, improved system reliability, and operational simplicity. Invarious optical WDM systems, integrated Coarse WDM (CWDM) or Dense WDM(DWDM) compact form pluggable (CFP) transceivers can be used to offer aneconomical and power-efficient way to implement 100-Gb/s transmission ona single fiber by an array of CWDM or DWDM lasers, each transmitting at10 Gb/s or 25 Gb/s aligning with CMOS electronic drive speeds.

SUMMARY

This specification describes, among others, techniques, devices andsystems for optical communications based on wavelength divisionmultiplexing that use tunable multi-wavelength laser transmittermodules.

In one aspect, a device for producing laser light at different opticalwavelengths is provided to include an array of tunable lasers to producelaser light at different optical wavelengths, respectively. Each tunablelaser includes a tunable sampled Bragg grating reflector responsive toan electrical control signal to produce tunable reflectivity peakswithin a tunable spectral range, a second grating reflector that isspaced from the sampled Bragg grating reflector to form an opticalresonator with the sampled Bragg grating reflector and producesreflectivity peaks at different second grating reflector resonancewavelengths within the tunable spectral range of the tunable sampledBragg grating reflector, and a gain section between the tunable sampledBragg grating reflector and the second grating reflector. The gainsection is capable of being electrically energized to produce an opticalgain for the laser light at a respective optical wavelength produced bythe tunable laser. This device includes a laser control unitelectrically coupled to the tunable lasers to apply the electricalcontrol signal as a common control signal to the tunable sampled Bragggrating reflector in each of the tunable lasers to synchronously tunethe tunable lasers that operate the different optical wavelengths,respectively.

In another aspect, a method for producing laser light at differentoptical wavelengths is provided to include operating an array of tunablelasers to produce laser light at different optical wavelengths,respectively, where each tunable laser includes a tunable sampled Bragggrating reflector responsive to an electrical control signal to tune arespective optical wavelength; and applying a common electrical controlsignal to the tunable lasers as the electrical control signal to eachtunable sampled Bragg grating reflector to synchronously tune thetunable lasers that operate at the different optical wavelengths,respectively.

In another aspect, a device for producing laser light at differentoptical wavelengths is provided to include a substrate and asemiconductor structure formed on the substrate and patterned to form anarray of tunable lasers to produce laser light at different opticalwavelengths, respectively, optical modulators located to receive andmodulate the laser light from the tunable lasers to carry information,respectively, and a beam combiner that receives laser light from theoptical modulators to produce a combined optical output. Each tunablelaser includes a tunable sampled Bragg grating reflector responsive toan electrical control signal to tune a respective optical wavelengthproduced by the tunable laser. This device includes a laser control unitelectrically coupled to the tunable lasers to apply a common electricalcontrol signal to the tunable lasers as the electrical control signal toeach tunable sampled Bragg grating reflector to synchronously tune thetunable lasers that operate at the different optical wavelengths,respectively.

In another aspect, a method for optical communications based onwavelength division multiplexing (WDM) includes providing tunable lasertransmitter modules based on a common tunable laser transmitter moduledesign which includes (1) a plurality of tunable laser modules toproduce laser light at different optical WDM wavelengths to carrydifferent WDM signal channels, respectively. Each tunable laser moduleis operable to tune a respective optical WDM wavelength, and (2) a beamcombiner that includes input ports that are respectively coupled toreceive the laser light from the tunable laser modules and an outputport that combines the received laser light from the tunable lasermodules into a combined optical output carrying light at the differentoptical WDM wavelengths. The beam combiner is structured to exhibit acommon transmission spectral profile from each input port to the outputport that has periodic repetitive transmission bands that are adjacentto one another in wavelength and extend to cover different WDMsub-bands. Each transmission band has a spectral width to cover apredetermined number of WDM wavelengths. In this method, the tunablelaser transmitter modules is operated to produce WDM optical signals atdifferent optical WDM wavelengths in different WDM sub-bands, onetunable laser transmitter module producing laser light at optical WDMwavelengths per WDM sub-band. The laser light at a respective opticalWDM wavelength from each tunable laser transmitter module is modulatedto produce a respective WDM optical signal carrying information of arespective WDM channel. This method uses a band multiplexer to receiveand to combine the WDM signals in the different WDM sub-bands from thetunable laser transmitter modules into a combined WDM optical output fortransmission over a fiber link.

In another aspect, a device for optical communications based onwavelength division multiplexing (WDM) includes a plurality of tunablelaser transmitter modules based on a common tunable laser transmittermodule design which includes (1) an array of tunable laser modules toproduce laser light at different optical WDM wavelengths to carrydifferent WDM signal channels, respectively, each tunable laser moduleoperable to tune a respective optical WDM wavelength, and (2) a beamcombiner that includes input ports that are respectively coupled toreceive the laser light from the tunable laser modules and an outputport that combines the received laser light from the tunable lasermodules into a combined optical output carrying light at the differentoptical WDM wavelengths. The beam combiner is structured to exhibit acommon transmission spectral profile from each input port to the outputport that has periodic repetitive transmission bands that are adjacentto one another in wavelength and extend to cover different WDM sub-bandsand each transmission band having a spectral width to cover apredetermined number of WDM wavelengths. This device includes a controlunit that controls the tunable laser transmitter modules to produce WDMoptical signals at different optical WDM wavelengths in different WDMsub-bands, respectively; and a band multiplexer coupled to receive andto combine the WDM optical signals in the different WDM sub-bands fromthe tunable laser transmitter modules into a combined WDM optical outputfor transmission over a fiber link.

In another aspect, a device for optical communications based onwavelength division multiplexing (WDM) includes an array of tunablelaser modules to produce laser light at different optical WDMwavelengths to carry different WDM signal channels, respectively andeach tunable laser module is operable to tune a respective optical WDMwavelength. This device includes a beam combiner that includes inputports that are respectively coupled to receive the laser light from thetunable laser modules and an output port that combines the receivedlaser light from the tunable laser modules into a combined opticaloutput carrying light at the different optical WDM wavelengths. The beamcombiner is structured to exhibit a common transmission spectral profilefrom each input port to the output port that has periodic repetitivetransmission bands that are adjacent to one another in wavelength andextend to cover different WDM sub-bands and each transmission bandhaving a spectral width to cover a predetermined number of WDMwavelengths. A control unit is included in this device to control thetunable laser modules to produce WDM optical signals at differentoptical WDM wavelengths, respectively.

In yet another aspect, a device for optical communications based onwavelength division multiplexing (WDM) includes tunable lasertransmitter modules to transmit WDM signals at different WDM sub-bands,respectively. Each tunable laser transmitter module produces laser lightof different WDM signals in a respective WDM sub-band and includes (1)tunable laser modules that produce laser light at different optical WDMwavelengths within a respective WDM sub-band to carry different WDMsignal channels within the respective WDM sub-band, respectively, eachtunable laser operable to tune a respective optical WDM wavelength, and(2) a beam combiner that includes input ports that are respectivelycoupled to receive the laser light from the tunable laser modules and anoutput port that combines the received laser light from the tunablelaser modules into a combined optical output carrying light at thedifferent optical WDM wavelengths. The beam combiner is structured toexhibit a common transmission spectral profile from each input port tothe output port that has periodic repetitive transmission bands that areadjacent to one another in wavelength and extend to cover the differentWDM sub-bands, wherein each transmission band has a spectral width tocover a predetermined number of WDM wavelengths. This device includes aband multiplexer that is coupled to receive WDM signals at the differentWDM sub-bands from the tunable laser transmitter modules and to combinethe received WDM signals into a combined WDM optical output fortransmission over a fiber link; a band splitter that is coupled toreceive an input WDM signal carrying input WDM signals at the differentWDM sub-bands and separates the WDM signals into different groups ofinput WDM signals within the different WDM sub-bands, respectively,along different optical paths; and optical receiver modules to receivethe different groups of input WDM signals at the different WDMsub-bands, respectively. Each optical receiver module includes (1) awavelength splitter that includes an input port coupled to receive arespective group of input WDM signals within a respective WDM sub-bandand output ports that output the received input WDM signals of therespective group along different output optical paths. The wavelengthsplitter is structured to exhibit a common transmission spectral profilefrom the input port to each output port that has periodic repetitivetransmission bands at are adjacent to one another in wavelength andextend to cover the different WDM sub-bands, and (2) optical detectorscoupled to receive the received input WDM signals of the respectivegroup and to convert the received input WDM signals into differentelectrical detector signals, respectively.

Particular embodiments of the invention can be implemented to realizeone or more of advantages. For example, a tunable transmitter module canuse tunable laser modules to produce tunable multi-wavelength opticalsignals and can be implemented to provide flexibility and simplicity inimplementation and inventory management. A tunable transceiver can beconfigured to achieve a high level of integration at a relatively lowcost.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a tunable multi-wavelength opticaltransceiver having tunable laser modules that respectively generatedifferent WDM signals at different WDM wavelengths.

FIG. 2 illustrates an example of tuning tunable lasers to different WDMwavelengths, respectively, that fall within a common transmission bandof the common transmission spectral profile of a cyclic beam combiner.

FIG. 3 shows an example of a WDM system by using the tunable transceiverin FIG. 1.

FIGS. 4A-B and 5 show a tunable sampled grating distributed Braggreflector (SG-DBR) laser and its tuning operation.

FIG. 6 shows one example of tuning an array of lasers based on the laserdesign in FIG. 4A.

FIG. 7 shows one implementation of a laser array of SG-DBR lasers inimplementing the tunable transmitter module in FIG. 1.

FIG. 8 shows one example for including semiconductor opticalamplification (SOA) function in a tunable optical transmitter ortransceiver module.

FIG. 9 shows another example for including an optical amplifier in atunable optical transmitter or transceiver module.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of a tunable multi-wavelength opticaltransceiver 100 having tunable laser modules 130 that respectivelygenerate different WDM signals at different WDM wavelengths. Thetransceiver 100 includes a tunable transmitter module 110 based on thetunable laser modules 130 to produce a WDM optical output 150 thatcarries multiple output WDM channels within a WDM spectral band, and areceiver module 120 that receives a WDM optical input 160 that carriesmultiple input WDM channels within the same WDM spectral band. Inimplementations, the tunable transceiver 100 in FIG. 1 can be configuredand packaged in various configurations to meet specific requirements ofWDM applications. For example, the transceiver 100 can be configured asa pluggable module based on the industry's multi-source agreement (MSA)for making a 40 GbE interface and a 100 GbE interface.

The tunable transmitter module 110 uses the tunable laser modules 130 toproduce tunable multi-wavelength optical signals 135 and can beimplemented to provide flexibility and simplicity in implementation andinventory management. The transceiver 100 can be configured to achieve ahigh level of integration in a single package at a relatively low costand to operate at a reduced power consumption. Multiple transceivers 100can be used in a WDM system and are tuned to produce WDM signals inmultiple WDM sub-bands within a WDM band and such WDM signals in themultiple WDM sub-bands are combined to generate a high throughput datarate, e.g., 100 Gb/s or higher.

The tunable transmitter module 110 in FIG. 1 includes tunable lasermodules 130 to transmit WDM signals 135 at different WDM wavelengths,respectively. The different WDM wavelengths can be WDM wavelengths basedon, e.g., the International Telecommunication Union (ITU) WDM gridspecification in the L, C and S bands for WDM applications. Each tunablelaser module 130 is operable to tune a respective optical WDM wavelengthto a desired value, e.g., an ITU WDM wavelength. In someimplementations, each tunable laser module 130 can include a tunablesemiconductor laser that directly produces modulated laser light as arespective WDM optical signal carrying information of a respective WDMchannel by modulating, e.g., a driving current applied to thesemiconductor laser. In other implementations, such as the specificexample illustrated in FIG. 1, each tunable laser module 130 can includea tunable laser 132, e.g., a semiconductor laser, that producescontinuous wave (CW) laser light at a WDM wavelength, and an opticalmodulator 134 that is located to receive the CW laser light from thetunable laser and modulates the CW laser light to produce the respectiveWDM optical signal 135 carrying information of the respective WDMchannel. Data 131 for the respective WDM channel is carried by amodulator control signal which is applied to the optical modulator 134.In response to the modulator control signal carrying the data 131, theoptical modulator performs the optical modulation on the CW laser light.The optical modulator 134 may be implemented in various configurations,such as an electro-optic Mach-Zehnder modulator (MZM) or anelectro-absorption modulator (EAM). FIG. 1 shows n WDM channels aregenerated at n different WDM wavelengths λ1, λ2, . . . , and λn. Acontrol unit is provided in the transceiver 100 to control and to tunethe tunable laser modules 130 to produce WDM optical signals at desireddifferent optical WDM wavelengths, respectively. This tunability of thetransceiver 100 allows the same transceiver 100 to be tuned to operatein different optical spectral bands such as different WDM sub-bandswithin a particular WDM band such as the C-band, L-band, S-band andother optical transmission bands for optical communications. In WDMsystem deployment, multiple identically constructed transceivers 100 canbe installed in the same transmit subsystem and tuned to produce WDMsignals in different WDM sub-bands. This aspect in the design of thetransceiver 100 eliminates the need for making different transceiversfor producing different sets of WDM signals in different WDM sub-bandsof a given WDM band and allows the same transceiver 100 to function astransceivers for different WDM sub-bands.

The transceiver 100 includes a beam combiner 140 that is locateddownstream from the tunable laser modules 130 to receive the WDM signals135 and combines the received WDM signals 135 at different optical WDMwavelengths into the combined optical output 150. The beam combiner 140includes input ports that are respectively coupled to receive the laserlight from the tunable laser modules 130 and a common output port thatoutputs the combined optical output 150. Notably, the beam combiner 140is structured to exhibit a common transmission spectral profile fromeach input port to the common output port that has periodic or cyclic,repetitive transmission bands that are adjacent to one another inwavelength and extend to cover different WDM sub-bands within a WDMband. Each transmission band has a spectral width to cover apredetermined number of WDM wavelengths. This cyclic spectral propertyof the beam combiner 140 combines with the tenability of the lasermodules 130 to provide the versatile operating capability of the tunabletransceiver 100 for producing different WDM signals in different WDMsub-bands.

FIG. 2 shows one exemplary implementation of the cyclic beam combiner140 in form of a cyclic arrayed-waveguide grating (AWG) multiplexer,where outputs of four tunable lasers 1-4 are respectively coupled tofour input ports 1-4 of the cyclic AWG multiplexer and are combined asthe WDM optical output at the AWG output port. The common transmissionspectral profile from each input port to the common output port is shownon the left side of each input port, illustrating three consecutivetransmission bands. Each of the transmission bands occupies a spectralwidth within the free spectral range (FSR) that defines the periodicityof the repetitive spectral response of the cyclical AWG multiplexer andcovers multiple WDM wavelengths. Such transmission bands can serve asthe WDM sub-bands within a particular WDM band, e.g., the C-band from1529 nm to 1565 nm, the L-band from 1565 nm to 1625 nm, the S-band from1460 nm to 1530 nm, and other bands.

The tunable lasers 1-4 can be tuned to 4 different WDM wavelengthswithin one or more of the transmission bands of the common transmissionspectral profile for each input port of the cyclic AWG multiplexer 140.Various tuning arrangements may be implemented.

FIG. 2 illustrates an example of tuning the tunable lasers to 4different WDM wavelengths, respectively, that fall within a commontransmission band of the common transmission spectral profile. Forexample, the four different WDM wavelengths λ1, λ2, λ3, and λ4 from thetunable lasers 1-4, respectively can be four consecutive WDM wavelengthsin the same transmission band as shown in FIG. 2. In the illustratedexample in FIG. 2, a particular tuning configuration is also shown inwhich each tunable laser is tuned at a coarse step by a change of oneFSR. Assuming the FSR covers m different WDM wavelengths or channels,the first tunable laser operating at λ1 can be tuned to the nextwavelength λ1+m) in the next transmission band. The four tunable lasers1-4 can be tuned in synchronization to their respective positions in thenext transmission band. Under this specific tuning design, each tunablelaser is tuned by tuning the laser frequency from one FSR to the nextFSR with an equal channel spacing of one FSR between the two consecutivetuned laser frequencies. For tuning within the C-band, for example, thespectral range of the C-band can be divided into spectral ranges knownas sub-bands corresponding to the transmission bands of the commontransmission spectral profile for each input port of the cyclic AWGmultiplexer 140. In tuning each tunable laser, the wavelength from thetunable laser is tuned to hop at a coarse step by one FSR from onetransmission sub-band to the next transmission sub-band. A common lasercontrol signal can be applied to the four tunable lasers 1-4 in a waythat synchronously tune the four tunable lasers 1-4 at the same time. Acontrol circuit can be used to generate the common laser control signaland is coupled to the lasers to apply the common laser control signal tothe lasers. In operation, the control circuit adjusts the common lasercontrol signal to synchronously tune all four lasers

Under the above tuning design, the FSR of the cyclic AWG multiplexer 140may be equal to n×Δf, where Δf is the channel spacing, n is the numberof tunable lasers in an array. Thus, an operating spectrum range can bedivided into

$l = \frac{spectrum}{n \times \Delta\; f}$number of sub bands, l is also the number of wavelengths that each laserin the array could be tuned to in the whole spectrum band. For example,for the whole C band (wavelength from 1529 nm to 1560 nm) with 100 GHz(0.8 nm) wavelength spacing, the number of sub bands that can beobtained is

$l = {\frac{32\mspace{14mu}{nm}}{n \times 0.8\mspace{14mu}{nm}}.}$

Under the above tuning design, when n tunable lasers are implemented inthe tunable transceiver, each laser is tuned discontinuously at a tuningstep of n×Δ. For example, assuming the WDM wavelength spacing is 100GHz, the tuning step is n×100 GHz (or n×0.8 nm). To cover the full rangeof a 32 nm C-band with a wavelength from 1529 nm to 1560 nm, awavelength spacing of 100 GHz (0.8 nm) and a number of 1 sub bands, thetuning range of each laser in a laser array may only be a ratio of(l−1)×(32/l) nm. When l=4, the tuning range of each laser needs to be 24nm instead of 32 nm.

In above example of the AWG multiplexer for implementing the beamcombiner 140 can be implemented in various AWG configurations, includinga cyclic Echelle grating multiplexer. Other devices can also be used toperform the function of the beam combiner 140. For example, the beamcombiner 140 can include a set of cascaded comb filters such asFabry-Perot filters with periodic frequency responses. In addition, awide-band n×1 power coupler, which has a loss of 10×log(n) dB, can beused to replace the cyclical AWG as the beam combiner 140.

Referring back to FIG. 1 for the tunable transceiver 100, the receivermodule 120 includes a beam splitter 170 that is wavelength selective andincludes an input port coupled to receive a WDM optical input 160containing input WDM signals within the same WDM band output by thetunable transmitter module 110. The beam splitter 170 includes outputports that output the received input WDM signals at different WDMwavelengths along different output optical paths. When the beam combiner140 is implemented to have the spectral properties of the cyclic AWGmultiplexer, the beam splitter 170 can structured to exhibit the same orsimilar common transmission spectral profile from the input port to eachoutput port as that in the beam combiner 140. The beam splitter 170 canbe implemented in various configurations, including a cyclic AWGdemultiplexer and a cyclic Echelle grating multiplexer. Different fromthe beam combiner 140 which may be a simple optical power combiner, thebeam splitter 170 is a wavelength-selective splitter that splits thereceived WDM signals at different WDM wavelengths into different opticalsignals at different WDM wavelengths along different output opticalpaths. Optical detectors 172 are coupled to receive the input WDMsignals and to convert the received input WDM signals into differentelectrical detector signals 175, respectively. RF signal amplifiers 174may be provided to amplify the signals 175.

The tunable transceiver 100 in FIG. 1 can be implemented in a fiber WDMtransmission system such as the example in FIG. 3 that uses a bank ofidentically constructed tunable transmitter modules 110 that are tunedto different WDM sub-bands of a WDM band. In a high-speed fiber WDMsystem, two or more tunable transceivers 100 in FIG. 1 can be used astunable laser transmitter modules to form a tunable multi-wavelengthtransceiver. The fiber WDM transmission system 300 in FIG. 3 shows onlythe transmitter subsystem on the transmitter side of the system 300 andthe receiver subsystem on the receiver side of the system 300 where atransmission fiber link 320 connects the two sides of the system 300. Acomplete tunable multi-wavelength transceiver subsystem includes boththe illustrated the transmitter subsystem and the receiver subsystem.More specifically, the complete tunable multi-wavelength transceiversubsystem on one side of the transmission fiber in FIG. 3 includestunable laser transmitter modules 110, a band combiner or multiplexer310, a band demultiplexer 320, and optical receiver modules 120.

The tunable laser transmitter modules 110 transmit WDM signals atdifferent WDM sub-bands within a WDM band, respectively. Each tunablelaser transmitter module 110 produces laser light of different WDMsignals in a respective WDM sub-band and has a structure shown inFIG. 1. The band multiplexer 310 is coupled to receive WDM signals atthe different WDM sub-bands from the tunable laser transmitter modules110 and combines the received WDM signals into a combined WDM opticaloutput for transmission over the transmission fiber 320. The banddemultiplexer 320 is coupled to receive an input WDM signal carryinginput WDM signals at the different WDM sub-bands from the transmissionfiber 320 and separates the WDM signals into different groups of inputWDM signals within the different WDM sub-bands, respectively, alongdifferent optical paths. Optical receiver modules 120 are provided toreceive the different groups of input WDM signals at the different WDMsub-bands, respectively. Assuming a complete tunable multi-wavelengthtransceiver subsystem includes there are M tunable transmitter modules110 in the transmitter subsystem and M receiver modules 120 in thereceiver subsystem, the total data transmission capacity of the systemisM×(n×m) Gb/swhere n is the number of tunable laser modules 130 in each tunabletransmitter module 110 and m is the data rate in Gb/s for each WDMchannel.

In implementing the system in FIG. 3, a 10×10 Gb/s Pluggable opticalmodule may be implemented to cover the whole C-band. The whole C bandspectrum from 1529 nm to 1560 nm offers a usable spectrum of about 4THz. This spectrum may be covered by four tunable Pluggable opticalmodules, with each covering a WDM sub-band occupying a quarter of theC-band spectrum. Each pluggable optical module may be implemented with a100-GHz space WDM to cover a 1-THz range or a quarter of the C-bandspectrum. The optical output of four different pluggable optical modulesoccupying different segments of the C-band spectrum can be furthersub-band multiplexed with an intermediate WDM (IWDM) shown as bandmultiplexer in FIG. 3 to pack the 4×10×10 G outputs onto a singletransmission path.

In actual deployment of the system 300 in FIG. 3, the basic transceiverbuilding block is the tunable transceiver 100 in FIG. 1. Multipleidentical tunable transceivers 100 in FIG. 1 are used in the completetunable multi-wavelength transceiver subsystem but are tuned to operatein different WDM sub-bands within a WDM band as shown in FIG. 3. Thereis no need to maintain different transceivers for operating in differentWDM sub-bands. Therefore, the design of the system 300 in FIG. 3 is toprovide flexibility and simplicity in implementation and inventorymanagement and to reduce the costs for constructing and maintaining thesystem 300.

The above described example of a tunable multi-wavelength transceiversubsystem in FIG. 3 can be integrated to provide a compact devicepackage. For example, the transmitter module 110 having tunable lasers132, optical modulators 134, the beam combiner 140, and other componentscan be monolithically or hybridly integrated. Similarly, in the receivepath, the receiver module 120 having the beam splitter 170, thephotodetectors 172, and signal amplifiers 174 can be monolithically orhybridly integrated. Various semiconductor laser designs can be used toachieve desired device integration. In some implementations, thetransmitter module 110 and the receiver module 120 can be completelyintegrated together as a single unit.

For example, the tunable laser 132 can be a tunable sampled gratingdistributed Bragg reflector (SG-DBR) laser and the array of SG-DBRlasers 132 in FIG. 1 can be integrated. FIG. 4A shows an example of aSG-DBR laser 400 for implementing the tunable laser 132 in FIG. 1.

In FIG. 4A, the SG-DBR laser 400 includes a substrate and asemiconductor structure formed on the substrate to include a firstsemiconductor layer 401 and a second semiconductor layer 403, and awaveguide layer 402 formed between the first and second semiconductorlayers 401 and 403. In some implementations, the first and secondsemiconductor layers 401 and 403 may have refractive indices less thanthe refractive index of the waveguide layer 402, e.g., by havingdifferent mixing ratio of compound materials with differentcompositions. The optical waveguide formed by the layers 401, 402 and403 confines and guides laser light generated by the SG-DBR laser 400along the optical waveguide. The semiconductor structure for the SG-DBRlaser 400 is structured to include a first sampled Bragg grating regionforming the rear reflector, and a second sampled Bragg grating regionspaced from the first sampled Bragg grating region and forming the frontreflector. A rear reflector electrode 410 is formed in the first sampleBragg grating region to apply an electrical signal S1 for tuning andcontrolling the Bragg resonance condition for the rear reflector, and afront reflector electrode 440 is formed in the second sample Bragggrating region to apply an electrical signal S4 for tuning andcontrolling the Bragg resonance condition in the front reflector. Eachsampled Bragg grating can be formed in or near the waveguide layer 402to interact with the laser light guided in the optical waveguide. Therear and front reflectors form a Fabry-Perot resonator as the lasercavity for the SG-DBR laser 400. An optically active gain region isformed between the rear and front reflectors to provide the optical gainfor the SG-DBR laser 400. The gain region is capable of beingelectrically energized to produce the optical gain and can be a quantumwell structure. A gain region electrode 430 is formed in the gain region432 to apply an electrical signal S3 to electrically excite the gainregion 432 to generate the laser light. Multiple lasers can be formed onthe same substrate.

The rear reflector can be configured as a high reflector to reflect thelaser light and the front reflector can be structured to form a partialreflector to the laser light to reflect part of the laser light backtowards the rear reflector and to transmit part of the laser light as alaser output 470 of the SG-DBR laser 400. A rear anti-reflective coating450 may be formed on the rear end facet of the semiconductor structurenear the rear reflector to reduce undesired optical feedback from therear end facet and a front anti-reflective coating 460 may be formed onthe front end facet of the semiconductor structure near the frontreflector to reduce undesired optical feedback from the front end facet.

The laser wavelength of the SG-DBR laser 400 is tuned by controllingeither or both of the front and rear reflectors via the control signalsS1 and S4 to operate at a laser wavelength where both a reflectivitypeak of the front reflector and a reflectivity peak of the rearreflectors align with each other. An electrical current is applied toeither or both the rear and the front reflectors to achieve wavelengthtuning by changing the refraction index and moving the reflectivitypeaks in wavelength. In some implementations, the electric current maybe applied to only one of the two reflectors in the SG-DBR laser 400,e.g., the rear reflector while maintaining the front reflect in a fixedBragg grating configuration, and therefore adjusting only the refractiveindex of one reflector while maintaining the refraction index of theother reflector at a constant.

The example of the SG-DBR laser 400 in FIG. 4A also shows a phasesection formed in the semiconductor structure between the first andsecond sampled Bragg grating reflectors that is electrically controlledto control a phase of the laser light inside the laser cavity. A phaseregion electrode 420 is formed to effectuate this phase section and isshown to be between the rear reflector and the gain region in thisexample. A phase control electrical signal S2 is applied via the phaseregion electrode 420 to adjust the refractive index of the phase sectionto change the phase of the laser light in providing a desired phasematching condition for sustaining the laser oscillation within the lasercavity.

The insert in FIG. 4A shows the structure of the sampled Bragg gratingfor the rear reflector that includes discrete sections of gratings witha grating period or pitch Λ which determines the reflectivity centerwavelength of the sampled grating, and a grating length L_(g) for eachgrating section which is related to the grating reflectivity. Thesampled Bragg grating has a sampling period length of L_(s) which is thespacing between the interrupted gratings or adjacent sections ofgratings. The optical spectrum of the reflection from the sampled Bragggrating reflector is a series of reflection peaks (as shown in FIG. 4B)at different wavelengths meeting the Bragg resonance condition and theseparation of the reflectivity peaks is:

${\Delta\lambda}_{s} = \frac{\lambda^{2}}{2n_{g}L_{s}}$where n_(g) is the effective group index of refraction of the waveguide.L_(s)=L/N_(s), where L is the total length of the sampled grating ineach reflector and N_(s) is the number of sampling periods.

FIG. 5 shows the comb-like reflection spectrum, known as the Verniereffect, achieved by the sampled grating of the rear reflector and thefront reflector. The comb-like reflection spectrum of the frontreflector is represented by the solid line and the reflection spectrumof the rear reflector is represented by the dotted line, both achievedby periodic sampling of the gratings of the front grating and reargrating.

The sampling periods of the front and rear reflectors may be chosen tohave a slight mismatch to produce two reflectivity-versus-wavelengthspectra with slightly different periodicities as shown in FIG. 5. Inthis situation, only one of reflection peaks in each of the two spectramay be aligned with the other, for example, the alignment as marked inFIG. 5. Thus the wavelength of the SG-DBR laser 400 may be controlled byaltering the alignment of the reflectivity peaks from the rear reflectorand the front reflector. In this way, a wide tuning range for the laserwavelength may be achieved.

The tuning range of an SG-DBR laser 400 is determined by the ratio ofsampling period L_(s) and grating length L_(g). Therefore longersampling period length L_(s) and shorter grating length L_(g) areadvantageous for achieving wide tuning range. In order to access all thewavelengths within a desired tuning range, the wavelength jump stepshould be sufficiently small to allow the gaps to be filled bysimultaneous tuning of the rear and the front reflectors of the tunableSG-DBR laser 400. For example, discrete tunable lasers built for a DWDMsystem with a 100 GHz wavelength spacing regularly require a fine jumpstep of 0.8 nm.

To reduce the step of wavelength jump, the sampling period length L_(s)should be increased, and the number of sampling periods N_(s) shouldalso be increased to obtain the same amount of power reflectivity whenthe sampling period length L_(s) increases. However, a greater opticalloss and an increase in device size would result from an increase in thesampling period length Ls and the number of sampling periods and areduction in the grating length. Furthermore, an integratedsemiconductor optical amplifier (SOA) outside the laser cavity on thesame substrate may be used to increase the optical power output of theSG-DBR 400. This addition of the SOA increases the size of the deviceand makes large-scale integration a challenge.

Such trade-offs in tuning range, tuning step, and optical performancemay be addressed by tuning at a coarse step and preferably by tuningonly one of the reflectors in the SG-DBR laser 400.

FIG. 6 shows a spectrum illustrating the coarse step tuning where thewhole band spectrum (for example, C-band or L-band) is divided intomultiple sub-bands from band-0 to band-1. The tunable SG-DBR laser 400can be tuned at a coarse step to hop from one sub-band to the nextsub-band. The frequency range of each sub band may be set to n×Δf, whereΔf is the channel spacing, n is the number of tunable lasers in anarray. Thus, an operating spectrum range can be divided into

$l = \frac{spectrum}{n \times \Delta\; f}$number of sub bands, l is also the number of wavelengths that each laserin the array could be tuned to in the whole spectrum band. For example,for the whole C band (wavelength from 1529 nm to 1560 nm) with 100 GHz(0.8 nm) wavelength spacing, the number of sub bands that can beobtained is

$l = {\frac{32\mspace{14mu}{nm}}{n \times 0.8\mspace{14mu}{nm}}.}$

Thus, for a laser array of n tunable lasers, each laser is tuneddiscontinuously at a tuning step of n∴Δf. For example, in FIG. 6 wherewavelength spacing is 100 GHz, the tuning step of a laser array of ntunable lasers is n×100 GHz (or n×0.8 nm). By tuning at a much coarserstep, sampling period length L_(s) in the SG-DBR laser 400 may beshortened and the optical power output is increased as a result ofreduced optical loss. As a result, an integrated SOA on the samesubstrate with the SD-DBR laser 400 may become optional and the SG-DBRdevice may therefore be made simpler and more compact.

In implementations where a laser array for an optical WDM transmittersuch as the example in FIG. 1 is formed of multiple SG-DBR lasers 400,each SG-DBR laser 400 in the laser array may be designed with adifferent grating pitch λ from other SG-DBR lasers 400 so the comb-likereflection spectra of the lasers 400 are shifted from one another by oneor more channel spacings of the WDM wavelengths which may follow the ITUWDM grid (e.g., 100 GHz or 50 GHz). Referring to FIG. 1 where eachtunable laser 132 is a SG-DBR laser 400 in FIG. 4A and further referringto FIG. 6, the first laser in the laser array for the WDM channel 1 maybe tuned to a central wavelength λ₁ in sub-band-0, and be tuned toλ_(n×Δλ+1) in the next band, band-1. The second laser for the WDMchannel 2 may have a wavelength λ₂ in sub-band-0 and hop to λ_(n×Δλ+2)in the next band, the third laser for the WDM channel 3 with awavelength λ₃ in sub-band-0 and λ_(n×Δλ+3) in the next band, and the nthlaser with a wavelength λ_(n) in sub-band-0 and λ_(n×Δλ+n) in the nextband. For example, in C-band, Δλ=0.8 nm for 100 GHz intra-sub-bandchannel spacing as shown in FIG. 6. In this scenario, each sub-band 0-lis filled by the wavelengths from the lasers in the laser array. One ofthe advantages of the above coarse tuning is that the laser modes arewidely separated and, accordingly, the mode suppression ratio (MSR) canbe improved.

FIG. 7 shows one implementation of a laser array of SG-DBR lasers 400 inimplementing the tunable transmitter module 110 in FIG. 1. The tunableSG-DBR circuit may be made compact in size by tuning only one of thefront or rear reflectors in each laser and thereby reducing a separatecontrol circuit for controlling the other reflector. In thisimplementation, only the sampled Bragg grating of the rear reflector ineach laser 400 is tuned while the sampled Bragg grating of the frontreflector that outputs the laser light to the optical modulator 134 ismaintained at a fixed Bragg grating configuration and is not tuned. Theelectrodes for the rear reflectors of all lasers 400 are connected to acommon control signal 701 from a common control unit or circuit 700 thatis used to simultaneously synchronously tune all rear reflectors in thelasers 400. The control unit 700 can be located within the same modulewhere the lasers 400 are in some implementations or be placed separatefrom the lasers 400. In an integrated design where the lasers 400 are onthe same chip, the control unit 700 may be integrated on the same chip.The control unit 700 controls the lasers 400 to produce laser light atdifferent optical WDM wavelengths in different WDM sub-bands,respectively and adjusts the common tuning control signal 701 tosynchronously tune all of the lasers 400 at the same time. Each laser400 in the array may be simultaneously tuned to each respectivewavelength in the next sub-band. By filling each sub-band withwavelengths from different laser sources and simultaneous tuning ofthese laser sources, a continuous tuning over the whole spectrum rangeis possible.

One of benefits for tuning only the rear reflector while maintaining thefront reflect at a fixed configuration is to decrease the front gratingtuning absorption loss and to obtain a higher optical output power fromthe front end of the SG-DBR laser 400. This design can increase theoverall optical output of the laser 400 and may ultimately to avoid theuse of a SOA for boosting the laser output. In implementations, thefront reflector may be designed with a fixed reflectivity spectrum and alow number of sampling periods to reduce the size of the laser 400. Thedesign of adjusting only the controlling current applied to the rearreflector can increase the precision of reflection alignment anddecrease the probability of mode jumps of the SG-DBR laser 400 toimprove the laser stability.

Alternative to the above SG-DBR laser 400, one of the two reflectors ofa tunable laser can be a fixed, non-tunable grating reflector formed ofa series of cascaded single-wavelength gratings to produce differentreflectivity peaks at different wavelengths, respectively while theother reflector is a tunable sampled Bragg grating. For example, thefront reflector can be the fixed grating reflector having a series ofcascaded single-wavelength gratings and the real reflector is a tunablesampled Bragg grating. The different reflectivity peaks of the cascadedsingle-wavelength gratings can be configured to be pre-aligned withvarious reflectivity peaks within the tuning spectral range of thetunable rear sampled Bragg grating to allow for tuning the tunable rearsampled Bragg grating to cause lasing at these reflectivity peaks.

While the laser array of the SG-DBR lasers 400 provides wide tuningrange for generating different WDM wavelengths in various WDM sub-bands,the tuning range of each individual SG-DBR laser 400 in the laser arrayis relatively small. For example, to cover the full range of a 32 nmC-band with a wavelength from 1529 nm to 1560 nm, a wavelength spacingof 100 GHz (0.8 nm) and a number of l sub bands, the tuning range ofeach laser in a laser array may only be a ratio of (l−1)×(32/l) nm. Whenl=4, the tuning range of each laser needs only to be 24 nm, as comparedto a 32 nm of a traditional SG-DBR. As explained above, the tuning rangeof a SG-DBR laser is determined by the ratio of sampling period lengthL_(s) and grating length L_(g), and L_(s) is the ratio of the totalsampled grating length L over the number of sampling periods N_(s).Therefore, when a tuning range decreases, the grating length L_(g) maybe increased and the number of sampling periods N_(s) may be reduced toprovide enough reflectivity. In this way, the SG-DBR laser arraystructure may be made more compact.

To compensate for transmission loss and reduce the spurious componentsinterfering other wavelengths when the laser is tuned from one band toanother, a SOA with suitable gain profiles may be provided outside thelaser 400 before and/or after the modulator 134 to increase the opticalcarrier signal level and/or the modulated output signal. Alternatively,due to various features described above, such as coarse tuning step,reduced sampling period length and increased output power in the presentdesign, the SOA may be selectively omitted from the circuit design.

FIG. 8 shows an example of a tunable SG-DBR laser with an integrated SOAin the same semiconductor structure where the laser is formed. A SOAregion 812 is formed in the waveguide outside the laser resonator formedby the two reflectors 410 and 440, e.g., between the front reflector 440and the front AR-coated facet 460. The output laser light from the fromthe front reflector of the laser is directed along the waveguide intothe SOA gain region 812 which is electrically energized by the signalapplied to the SOA electrode 810 to produce the optical gain thatamplifies the output laser light as the amplified output laser light820.

In some applications, a common optical amplifier can be used to amplifylight from the array of the tunable lasers. FIG. 9 shows an example ofan optical transmitter module that includes an optical amplifier at theoutput of the beam combiner 140. This amplifier can be an SOA and can beintegrated to the semiconductor structure in which the lasers 400, theoptical modulators 134 and the cyclic AWG combiner 140 are formed. Thissingle SOA is shared among all the individual wavelengths in the WDMsub-band.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of theinvention or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of the invention. Certainfeatures that are described in this specification in the context ofseparate embodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments.

Thus, particular embodiments of the invention and their implementationsare disclosed. Variations, modifications and enhancements of thedisclosed embodiments and implementations and other embodiments andimplementations can be made based on what is described and illustratedin this specification.

What is claimed is:
 1. A device for producing laser light at differentoptical wavelengths, comprising: an array of tunable lasers that eachemit laser light that is a channel spacing wavelength away from thelaser light of an adjacent tunable laser in the array, wherein eachtunable laser in the array includes: a tunable sampled Bragg gratingreflector for producing laser light having reflectivity peaks within atunable spectral range, wherein the tunable sampled Bragg gratingreflector is coupled to be tuned by a common control signal; a secondgrating reflector that is spaced from the tunable sampled Bragg gratingreflector to form an optical resonator with the tunable sampled Bragggrating reflector; and a gain section between the tunable sampled Bragggrating reflector and the second grating reflector, the gain sectioncapable of being electrically energized to amplify the laser light; anda laser control unit coupled to control the common control signal tosynchronously tune the tunable sampled Bragg grating reflector of eachtunable laser from a first sub-band within a first band of the tunablespectral range to a second sub-band within a second band of the tunablespectral range, wherein the first sub-band is a coarse tuning step fromthe second sub-band and the coarse tuning step is approximately equal toa number of the tunable lasers in the array multiplied by the channelspacing wavelength.
 2. The device as in claim 1, wherein: the secondgrating reflector includes a series of fixed optical gratings thatrespectively reflect light at different second grating reflectorresonance wavelengths that are fixed in wavelength and are not tunable.3. The device as in claim 1, wherein: the second grating reflectorincludes a second sampled Bragg grating reflector.
 4. The device as inclaim 3, wherein: sampling periods of the sampled Bragg gratingreflector and the second sampled Bragg grating reflector are different.5. The device as in claim 3, wherein: a number of sampling periods inthe sampled Bragg grating reflector is higher than a number of samplingperiods in the second sample Bragg grating reflector to output the laserlight out of a respective tunable laser through the second sample Bragggrating reflector.
 6. The device as in claim 1, wherein: each tunablelaser includes a phase section with the optical resonator that iselectrically controlled to control a phase of the laser light.
 7. Thedevice as in claim 1, wherein: the sampled Bragg grating reflector has asampling period greater than a grating period.
 8. The device as in claim1, comprising: a substrate; and a semiconductor structure formed on thesubstrate and patterned to form the tunable lasers.
 9. The device as inclaim 1, wherein: each tunable laser produces continuous wave (CW) laserlight at a respective optical wavelength; and the device comprises anarray of optical modulators that are located outside the opticalresonators of the tunable lasers to receive the CW laser light from thetunable lasers, respectively, each optical modulator modulatingrespective CW laser light from a respective tunable laser to produce amodulated optical signal carrying information.
 10. The device as inclaim 1, wherein: each tunable laser produces continuous wave (CW) laserlight at a respective optical wavelength; and the device comprises anarray of optical modulators that are located outside the opticalresonators of the tunable lasers to receive the CW laser light from thetunable lasers, respectively, each optical modulator modulatingrespective CW laser light from a respective tunable laser to produce amodulated optical signal carrying information.
 11. The device as inclaim 1, wherein: each tunable laser directly produces modulated laserlight carrying information.
 12. The device as in claim 1, comprising: anarray of optical amplifiers that are located to receive, respectively,the laser light from the tunable lasers, each optical amplifieramplifying the laser light received from a respective tunable laser. 13.The device as in claim 12, comprising: a semiconductor structurepatterned to form the respective tunable laser, wherein a respectiveoptical modulator is formed in the semiconductor in which the respectivetunable laser is formed.
 14. The device as in claim 1, wherein: thelaser control unit synchronously tunes the different optical wavelengthsof the tunable lasers discretely by a common amount of change inwavelength without changing a relative spacing between the differentoptical wavelengths.
 15. The device as in claim 1, comprising: a beamcombiner coupled to receive the laser light from the tunable lasers atdifferent optical wavelengths to produce a combined optical output; andan optical amplifier coupled to receive the combined optical output fromthe beam combiner and amplifying the combined optical output.
 16. Adevice for producing laser light at different optical wavelengths,comprising: a substrate; a semiconductor structure formed on thesubstrate and patterned to form an array of tunable lasers to producelaser light at the different optical wavelengths that are separated by achannel spacing, respectively, each tunable laser including a tunablesampled Bragg grating reflector responsive to a control signal to tune arespective optical wavelength produced by the tunable laser, wherein asampling period length of the tunable sampled Bragg grating reflectorcreates a discontinuous wavelength tuning step from a first sub-bandwithin a first band to a second sub-band within a second band, whereinthe discontinuous wavelength tuning step is approximately equal to anumber of the tunable lasers in the array of tunable lasers, multipliedby, a channel spacing between the different optical wavelengths of thetunable lasers in the array; and a laser control unit coupled to thetunable lasers to apply a common control signal to the tunable lasers asthe control signal to each tunable sampled Bragg grating reflector tosynchronously discontinuously tune the tunable lasers that operate atthe different optical wavelengths, respectively.
 17. The device as inclaim 16, wherein: the laser control unit tunes the different opticalwavelengths of the tunable lasers discretely by a common amount ofchange in wavelength without changing a relative spacing between thedifferent optical wavelengths.
 18. The device as in claim 16, wherein:the laser control unit tunes the different optical wavelengths of thetunable lasers discretely by a common amount of change in wavelengthequal to a spectral width of each transmission band of a commontransmission spectral profile without changing a relative spacingbetween the different optical wavelengths.
 19. The device as in claim18, comprising: an optical amplifier formed in the semiconductorstructure to receive and amplify a combined optical output from a beamcombiner.
 20. The device as in claim 18, comprising: optical amplifiersformed in the semiconductor structure and located to receive and amplifythe laser light from the tunable lasers, respectively.